Grating based optical coupler

ABSTRACT

An apparatus including a waveguide region configured to guide light propagating along a first direction; a reflector region configured to reflect incident light; an interference region formed between the waveguide region and the reflector region, the interference region configured to confine at least a portion of interference light formed by the incident light and the reflected incident light; and a grating region including a grating formed on a region confining at least a portion of the interference light, the grating configured to couple at least a portion of the light along a second direction that is different from the first direction.

CROSS-REFERENCE TO RELATED APPLICATIONS

This patent application is a continuation of U.S. patent applicationSer. No. 14/510,799, filed Oct. 9, 2014, which claims the benefit ofU.S. Provisional Patent Application No. 62/014,182, filed Jun. 19, 2014,U.S. Provisional Patent Application No. 62/012,446, filed Jun. 16, 2014,U.S. Provisional Patent Application No. 61/979,489, filed Apr. 14, 2014,U.S. Provisional Patent Application No. 61/946,657, filed Feb. 28, 2014,U.S. Provisional Patent Application No. 61/925,629, filed Jan. 9, 2014,and U.S. Provisional Patent Application No. 61/895,493, filed Oct. 25,2013 which are incorporated by reference herein.

BACKGROUND

This specification relates to coupling light using a grating.

Light propagates inside a photonic integrated circuit, and is coupled toan external medium through a grating fabricated on the photonicintegrated circuit.

SUMMARY

According to one innovative aspect of the subject matter described inthis specification, light may be generated, guided, processed, ordetected by one or more active and/or passive optical components of aphotonic integrated system. A grating may be formed on an interferenceregion of the photonic integrated circuit, where the interference regionmay be bounded by one or more reflectors. By matching a grating profilewith an interference pattern produced by the one or more reflectors,light may reciprocally enter or exit the photonic integrated circuit ata specific angle from or to an external medium, respectively.

In general, one innovative aspect of the subject matter described inthis specification can be embodied in an optical apparatus that includesa waveguide region configured to guide light propagating along a firstdirection; a reflector region configured to reflect incident light; aninterference region formed between the waveguide region and thereflector region, the interference region configured to confine at leasta portion of interference light formed by the incident light and thereflected incident light; and a grating region including a gratingformed on a region confining at least a portion of the interferencelight, the grating configured to couple at least a portion of the lightalong a second direction that is different from the first direction.

This and other implementations can each optionally include one or moreof the following features. The second direction may be substantiallyperpendicular to the first direction. The waveguide region may include asecond reflector region configured to (i) reflect, by a particularreflectivity, a portion of incident light, and (ii) transmit anotherportion of the incident light. Light may be attenuated based on anone-circulation attenuation coefficient after propagatingone-circulation in the interference region. The reflectivity of thesecond reflector region may be configured to substantially match theone-circulation attenuation coefficient. A portion of the grating regionmay be formed on the second reflector region.

The optical apparatus may include an n-doped region and a p-doped regionconfigured to provide an electric field in the second reflector regionwith an application of a voltage or a current across the n-doped regionand the p-doped region, where the second reflector region may beconfigured to provide a different reflectivity with the application of avoltage or a current across the n-doped region and the p-doped region.

The optical apparatus may include an n-doped region and a p-doped regionconfigured to provide an electric field in the reflector region with anapplication of a voltage or a current across the n-doped region and thep-doped region, where the reflector region may be configured to providea different reflectivity with the application of a voltage or a currentacross the n-doped region and the p-doped region.

The optical apparatus may include an n-doped region and a p-doped regionconfigured to provide an electric field in the interference region withan application of a voltage or a current across the n-doped region andthe p-doped region, where the interference region may be configured toprovide a different interference pattern for the interference light withthe application of a voltage or a current across the n-doped region andthe p-doped region.

The optical apparatus may include a light source region configured togenerate light, wherein the light source region is coupled to theinterference region, the waveguide region, or the reflector region. Theoptical apparatus may include an optical package having an apertureabove the grating region, the aperture configured to transmit lightalong the second direction.

The optical apparatus may include a light absorption region configuredto absorb at least a portion of light, wherein the light absorptionregion is coupled to the interference region, the waveguide region, orthe reflector region.

The grating region may include a two-dimensional grating configured toseparate light into two light portions propagating along two directions,or combine two light portions propagating along two directions into onelight portion.

A second grating region may be formed on a side of the interferenceregion that is at a different side to the grating region, the secondgrating region configured to couple at least a portion of the lightalong a third direction. The reflector region may include one of acorner mirror, a distributed Bragg reflector (DBR) mirror, a dispersivemirror, a waveguide loop mirror, or a metal layer. A grating periodicityof the grating region may substantially match an interferenceperiodicity of the light inside the interference region. The grating mayhave lattice vectors formed so that the locations of the in-phaseantinodes of the light inside the interference region substantiallymatch the locations of the grating valleys or peaks. A portion of thegrating region may be formed on the interference region, the reflectorregion, or the waveguide region.

Another innovative aspect of the subject matter described in thisspecification can be embodied in an optical apparatus that includes alight source region configured to generate photons; a first reflectorregion and a second reflector region; an interference region formedbetween the first reflector region and the second reflector region andcoupled to the light source region, and the interference regionconfigured to (i) guide the incident light generated by the light sourceregion to propagate along a first direction, and (ii) confineinterference light formed by light reflected between the first reflectorregion and the second reflector region along the first direction; and agrating region including a grating formed on a region confining at leasta portion of the interference light, the grating configured to emit aportion of the interference light along a second direction that isdifferent from the first direction.

This and other implementations can each optionally include one or moreof the following features. The effective refractive index of theinterference region may be lower than the effective refractive index ofthe grating region.

Another innovative aspect of the subject matter described in thisspecification can be embodied in an optical apparatus that includes afirst reflector region and a second reflector region; an interferenceregion formed between the first reflector region and the secondreflector region, the interference region configured to confineinterference light formed by light reflected between the first reflectorregion and the second reflector region propagating along a firstdirection; and a light absorption region coupled to the interferenceregion, the light absorption region configured to absorb at least aportion of the interference light; and a grating region comprising agrating formed on a region confining at least a portion of theinterference light, the grating configured to receive, from an externalmedium, light propagating along a second direction that is differentfrom the first direction.

This and other implementations can each optionally include one or moreof the following features. The light absorption region and the gratingregion may have different material composition, where the grating regionmay include metal forming surface plasmonic effect that couples lightpropagating along the second direction to the light absorption region.

Advantageous implementations may include one or more of the followingfeatures. Light may be coupled in or out of a photonic integratedcircuit at an angle that is substantially perpendicular to thepropagation direction of the light inside the photonic integratedcircuit. This perpendicularity may reduce packaging cost and complexity.Reflected light back to the photonic integrated circuit may be minimizedby one or more mirrors in the interference region to maintain stabilityof the photonic integrated circuit. The optical mode profile of thelight exiting the grating may be shaped to match the optical modeprofile of the external medium to minimize mode matching loss. Since theelectric field in the interference region is quite uniform, there is noneed for a chirped grating to match a traveling wave causedexponentially-decayed field profile. The interference region or gratingmay be actively tuned by mechanisms including electrical field, magneticfield or mechanical movement, to control the coupling of light. When theinterference region is coupled to an active medium that generates lightat a broad wavelength range, the interference region may be used toselect a narrower wavelength range from the broad wavelength range. Whenthe interference region is coupled to an absorption medium that detectslight, the interference region may be used to increase the absorptionefficiency through multiple reflections inside the interference region.

Other implementations of this and other aspects include correspondingsystems, apparatus, and computer programs, configured to perform theactions of the methods, encoded on computer storage devices. A system ofone or more computers can be so configured by virtue of software,firmware, hardware, or a combination of them installed on the systemthat in operation cause the system to perform the actions. One or morecomputer programs can be so configured by virtue of having instructionsthat, when executed by data processing apparatus, cause the apparatus toperform the actions.

The details of one or more implementations are set forth in theaccompanying drawings and the description below. Other potentialfeatures and advantages will become apparent from the description, thedrawings, and the claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a block diagram of an example photonic integrated circuit.

FIGS. 1 B, 1C, and 1D show examples of an optical coupler.

FIG. 2 is an example of interference patterns.

FIGS. 3A-3E show examples of a grating pattern.

FIGS. 4A-4F show examples of an optical coupler integrated with a lightsource.

FIGS. 5A-5C show examples of an optical coupler integrated with a lightdetector.

FIG. 6 shows an example of an optical coupler integrated with p-njunctions.

FIGS. 7A-7B show examples of an optical coupler having multiple outputpaths.

FIG. 8 shows an example of an optical coupler for packaging.

FIGS. 9A-9E show examples of a mirror.

FIG. 10 shows an example of a flow diagram for designing an opticalcoupler.

FIG. 11 shows an example of a flow diagram for fabricating an opticalcoupler.

Like reference numbers and designations in the various drawings indicatelike elements. It is also to be understood that the various exemplaryembodiments shown in the figures are merely illustrative representationsand are not necessarily drawn to scale.

DETAILED DESCRIPTION

FIG. 1A is a block diagram of an example photonic integrated circuit 100that includes an grating-based optical coupler for enabling the couplingof light into and out of a photonic integrated circuit 100. In general,an optical coupler with substantial vertical emission is useful forinterfacing surface-emitting/receiving optoelectronic devices, and canreduce the packaging cost and complexity due to off-normalconfiguration.

The photonic integrated circuit 100 includes one or more opticalcomponents fabricated on a substrate 116. The optical components includea waveguide region 102, a first reflector region 106, an interferenceregion 110, a second reflector region 114, and a grating region 120. Thesubstrate 116 may be any type of substrate that is suitable forfabricating a photonic integrated circuit. For example, the substrate116 may be a silicon wafer, a silicon-on-insulator (SOI) wafer, a III-Vmaterial such as a gallium arsenide (GaAs) or an indium phosphide (InP)wafer, or a glass wafer. As another example, the substrate 116 may be alayer of passive or active material deposited over an integratedelectronic circuit. As another example, the substrate 116 may be a layerof passive or active material deposited over another integrated photoniccircuit.

In general, the waveguide region 102 is configured to confine lightalong one or more dimensions to guide light along a specific direction.In some implementations, the waveguide region 102 may confine lightalong one dimension. For example, the waveguide region 102 may be a slabwaveguide that confines light along the z direction. In someimplementations, the waveguide region 102 may confine light along twodimensions. For example, the waveguide region 102 may be a rib waveguideor a channel waveguide that confines light along the y and z directions,such that the light may propagate along the x direction, as designatedby the arrow 122. The term “along the x direction” and their derivativesas used herein may be used to represent bidirectional (±x direction), orunidirectional (+x, -x). Furthermore, when light is traveling inside amulti-mode fiber placed along the x direction, some portions of thelight may travel in zigzag routes inside the fiber while the overalldirection may still be regarded as along the x direction.

In general, the first reflector region 106 and the second reflector 114are configured to reflect incident light. For example, when light in thewaveguide region 102 is incident on an interface 104, some portion ofthe light may be reflected back to the waveguide region 102, while theremaining portion of the light may be transmitted to the first reflectorregion 106. Similarly, when light in the first reflector region 106 isincident on an interface 108, some portion of the light may bereflected, while the remaining portion of the light may be transmittedto the interference region 110. Similarly, when light in theinterference region 110 is incident on an interface 112, some portion ofthe light may be reflected, while the remaining portion of the light maybe transmitted to the second reflector region 114. In someimplementations, a reflector may be an interface between two mediumswith different refractive indices.

The portion of the light that is reflected by a reflector may range fromnear zero percent to near one hundred percent, depending on the design.In some implementations, the first reflector region 106 or the secondreflector 114 may be highly reflective. For example, the secondreflector 114 may be coated with a metal, such as aluminum, to achieve ahigh reflectivity. As another example, light may be arrange to beincident on the second reflector 114 beyond a critical angle, where thelight is reflected through total internal reflection. As anotherexample, the second reflector 114 may be a Bragg reflector that provideshigh reflectivity for a range of wavelengths. As another example, thefirst reflector region 106 may comprise one or multiple slitsdisconnecting the waveguide region 102 and the interference region 110.As another example, the first reflector region 106 may comprise DBRstructure. As another example, the first reflector region 106 may be ananomalous dispersive mirror having multiple high/low refractive indexstructures in the x direction to compensate the wavelength-dependentoptical phase of the interference region and achieve a wider bandwidthof operation.

In some implementations, the first reflector region 106 or the secondreflector 114 may be partially transmissive and partially reflective.For example, the first reflector region 106 may be configured to (i)reflect, by a particular reflectivity, a portion of incident light, and(ii) transmit another portion of the incident light. A partialreflective reflector may be implemented, for example, by depositing adielectric material having a refractive index that is lower than thematerial of the waveguide region 102 in the corresponding reflectorregion. The percentage of reflected and transmitted light may becalculated using Fresnel equations.

In general, the interference region 110 acts as a cavity having a cavitylength, L_(Cavity), that is formed between the waveguide region 102 andthe second reflector region 114. In some implementations, the firstreflector region 106 may be formed between the waveguide region 102 andthe interference region 110, where L_(Cavity) may be defined as thelength between the first reflector region 106 and the second reflectorregion 114. In some implementations, an effective refractive index ofthe waveguide region 102 may be substantially equal to an effectiverefractive index of the interference region 110. For example, both thewaveguide region 102 and the interference region 110 may be fabricatedin silicon with the same cross sectional waveguide dimensions along they-z dimensions. In this case, the effective refractive index of thewaveguide region 102 is equal to an effective refractive index of theinterference region 110. As another example, both the waveguide region102 and the interference region 110 may be fabricated in silicon, butthe cross sectional waveguide dimensions along the y-z dimensions mayvary, which may cause a difference between the effective refractiveindex of the waveguide region 102 and the effective refractive index ofthe interference region 110. In this case, the effective refractiveindex of the waveguide region 102 is treated as substantially equal tothe effective refractive index of the interference region 110 as long asa resulting performance degradation, such as an optical loss, caused bythe effective refractive index difference is within a range that isacceptable for a targeted application.

The interference region 110 is configured to confine an interferencelight formed by an incident light and a reflected incident light. Forexample, a standing wave pattern between the first reflector 106 and thesecond reflector 114 may be formed in the interference region 110. Tohave interference formed in the interference region 110, the length ofthe cavity L_(Cavity) and the parameters of grating region 120 arechosen so that the incident light may reach to and get reflected by thesecond reflector 114 without being totally attenuated in the first passpropagating from the first reflector 106 to the second reflector 114. Insome implementations, the confinement may be a partial confinement,where a portion of the interference light is transmitted through thefirst reflector 106 back to the waveguide region 102 and/or a portion ofthe interference light is transmitted through the second reflector 114.The interference of light formed by the incident light and the reflectedincident light is described in more details in FIG. 2.

In some implementations, an optical path length of the interferenceregion 110 may be longer than a wavelength of the guided light. In someother implementations, an optical path length of the interference region110 may be shorter than the wavelength of the guided light. For example,for an interference region 110 composed of silicon having a cavitylength of 0.4 μm and a refractive index of 3.45, the interference region110 has an optical path length of 0.4 μm×3.45=1.38 μm. If the wavelengthof the guided light has a wavelength of 1.55 μm, the optical path lengthof the interference region 110 is shorter than the wavelength of theguided light. In this case, the light having the 1.55 μm wavelength maybe coupled to the grating region 120 through an evanescent field of thelight confined (partial confinement) in the interference region 110.

In general, the grating region 120 having a grating length of,L_(Grating), is configured either to couple at least a portion of thelight in a photonic integrated circuit 100 to an external medium 130, orto couple at least a portion of the light from the external medium 130to the photonic integrated circuit 100. In some implementations, thegrating length L_(Grating) may be shorter than the cavity lengthL_(Cavity). In some other implementations, the grating lengthL_(Grating) may be equal to the cavity length L_(Cavity). In some otherimplementations, the grating length L_(Grating) may be longer than thecavity length L_(Cavity). For example, the grating region 120 may befabricated on the interference region 110, but a portion of the gratingregion 120 may be extended into the waveguide region 102, and/or thefirst reflector region 106, and/or the second reflector region 114. Asused in this specification, the grating is formed or fabricated on aregion means that the grating is formed above the region, or the gratingis embedded at least partially inside the region. For example, thegrating may be formed by etching into the region which the grating isdisposed over.

In some implementations, the interference region 110 and the gratingregion 120 may have the same material composition. For example, thegrating region 120 may be fabricated by etching a grating patterndirectly on the surface of the interference region 110. In some otherimplementations, the interference region and the grating region may havedifferent material compositions. For example, the grating region 120 maybe fabricated by depositing silicon dioxide on the surface of asilicon-based interference region 110. A grating pattern may then beetched on the surface of the silicon dioxide to form an oxide grating.As another example, the grating region 120 may be fabricated bydepositing metal on the surface of the interference region 110 and thenbe etched to form a metal grating. As another example, the gratingregion 120 may be fabricated by depositing a higher refractive indexmaterial on the surface of the interference region 110 having a lowerrefractive index to improve the grating efficiency by attracting theoptical mode toward the grating side. The lower refractive indexmaterial can for example be InP and the higher refractive index materialcan for example be Si.

In general, the grating region 120 redirects light propagating along afirst direction to a second direction that is different from the firstdirection. In some implementations, the grating region 120 may redirectlight propagating along a first direction to a second direction that issubstantially perpendicular to the first direction. For example, bysubstantially matching a grating periodicity of a grating region 120 tothe interference periodicity of a standing wave pattern in theinterference region 110, the grating region 120 may redirect the lightpropagating inside the waveguide region 102 along the x direction, asdesignated by the arrow 122, to a perpendicular direction along the zdirection, as designated by the arrow 123. The term “substantiallymatch” as used in this application means that a resulting performancedegradation, such as an optical loss, due to the mismatch is within anacceptable range for a targeted application. An acceptable range may be,for example, within an order of magnitude. In some otherimplementations, the grating region 120 may redirect light propagatingalong the first direction to a second direction that is notsubstantially perpendicular to the first direction. The term“substantially perpendicular” as used in this application means 90degrees with an error margin that is acceptable for a targetedapplication.

The external medium 130 may be any medium that can transmit, guide,detect, or generate light. For example, the external medium 130 may bean optical fiber. As another example, the external medium 130 may be aphoto-detector. As another example, the external medium 130 may be alight source. In some implementations, a cladding 124 may be formedbetween the grating region 120 and the external medium 130. The cladding124 may be formed to protect the photonic integrated circuit 100, or toprovide a specific distance between the grating region 120 and theexternal medium 130. In some implementations, a cross-sectional modeprofile of the light emitted from the grating region 120 may be designedto substantially match a cross-sectional mode profile of the externalmedium 130 configured to receive the light emitted from the gratingregion. For example, the cross-sectional mode profile of the lightemitted from the grating region 120 in the x-y dimensions may bedesigned to substantially match the cross-sectional mode profile of thesingle mode fiber in the in the x-y dimensions.

FIG. 1B shows an example of an optical coupler 101 that may beimplemented in the photonic integrated circuit 100. The optical coupler101 may also be implemented in any one of the other photonic integratedcircuits described throughout this application, or in another photonicintegrated circuit not described in this application.

The optical coupler 101 includes an interference region 110 and agrating region 120. The grating region 120 includes grating valleys 118and grating peaks 126, which together form a grating having a gratinglength, L_(Grating). The height difference between a grating peak 126and a grating valley 118 determines a grating height. The ratio of thepeak width to the sum of peak width and valley width of the gratingalong the wave propagation direction determines the duty cycle of thegrating. The sum of the grating peak width and the grating valley widthdetermines the period of the grating. By tuning the grating height, theduty cycle, the grating period, the grating shapes, the claddingcovering the grating, or a combination of the above, the directionalityand far field angle of light emitted/received by the grating region 120can be determined. For example, the grating height and the duty cyclemay be modified to optimize the directionality of the light. As anotherexample, the grating period and the duty cycle may be tuned to achievethe desired far field angles which might be most suitable for a targetedapplication.

In some implementations, the height of the grating peaks may be higherthan the height of the first reflector region 106 and/or the secondreflector region 114. For example, the first reflector region 106, theinterference region 110, and the second reflector region 114 may beplanarized by polishing, and then another layer of material is depositedon the planarized surface so that the grating region 120 may be formedby patterning and etching.

In some other implementations, the height of the grating valleys may belower than the height of the first reflector region 106 and/or thesecond reflector region 114. FIG. 1C shows an example of an opticalcoupler 103 where the height of the grating valleys 119 is lower thanthe height of the first reflector region 106 and the second reflectorregion 114. For example, the first reflector region 106, theinterference region 110, and the second reflector region 114 may beplanarized by polishing, and then the grating region 120 may be formedon the polished surface by patterning and etching the interferenceregion 110. The optical coupler 103 may be implemented in the photonicintegrated circuit 100. The optical coupler 103 may also be implementedin any one of the other photonic integrated circuits describedthroughout this application, or in another photonic integrated circuitnot described in this application.

FIG. 1D shows an example of an optical coupler 105 that includes thewaveguide region 102, the interference region 110, the grating region120, and the second reflector region 114, but does not include the firstreflector region 106. The boundary 130 between the waveguide region 102and the interference region 110 is denoted by a dashed line 130 becausein some implementations, the waveguide region 102 and the interferenceregion 110 are composed of the same material or having substantiallyequal effective refractive index.

The optical coupler 105 does not include the first reflector region 106in the case where light in the interference region 110 is attenuatedbelow a threshold value after propagating one-circulation in theinterference region 110. For example, a standing wave may be created inthe interference region 110 through interference between forward lightincident on the second reflector region 114 and backward light reflectedby the second reflector region 114. The standing wave may diminish nearthe boundary 130 between the waveguide region 102 and the interferenceregion 110 because the light reflected by the second reflector region114 is attenuated below the threshold value beyond the boundary 130. Thethreshold value, for example, may be less than 10% of the initialincident light power. By substantially matching a grating pattern in thegrating region 120 with the standing wave pattern in the interferenceregion 110, the optical coupler 105 can be used to redirect lightpropagating along a first direction to a second direction that isdifferent to the first direction without the first reflector region 106.For example, the optical coupler 105 may be used to direct light to asecond direction that is substantially perpendicular to the firstdirection. In some implementations, without the first reflector region106, the optical coupler can still redirect an incident light with highefficiency if the light is attenuated below a threshold value afterpropagating one-circulation in the interference region 110. In someimplementations, to maintain high efficiency without introducing thefirst reflector region 106, the grating region 120 needs to provideenough one-circulation attenuation. For example, the grating lengthL_(Grating) needs to be long enough to provide sufficientone-circulation attenuation before reaching the boundary 130.

FIG. 2 is an example of a grating pattern 207 that substantially matchesan interference periodicity of the standing wave 205 inside theinterference region. The descriptions of FIG. 2 may be applied to anyone of the optical couplers described in this application. Generally,the round-trip phase shift is the sum of the phase shift introduced bythe one-circulation propagation and the phase shift introduced by areflector. To simplify the description, the phase shift introduced bythe reflector may be assumed to be zero so that the resonant condition“round-trip phase shift equals 2mπ” can be viewed as the same as“one-circulation phase shift equals 2mπ,” where m is any integer.

In some implementations, light propagating in a waveguide may beconfined by two dimensions. For example, referring to FIG. 1A, lightpropagating in the waveguide region 102 is confined in y and zdimensions. When the light enters an interference region, theconfinement of the waveguide may be weakened, and the light propagateslike a point wave inside the interference region. For example, theinterference region 110 may be designed to confine light tightly in thez dimension and confine light loosely in the y dimension. The point wavereaches a reflector 211, gets reflected, and a standing wave pattern 205having a periodicity of dl may be formed inside the interference regionthrough the interference of a forward propagating wave 201 and abackward propagating wave 203.

In some implementations, a grating pattern 207 may be designed tosubstantially match the standing wave pattern 205. By matching thestanding wave pattern 205, this grating pattern 207 may act as anoptical antenna and become the most efficient way for light to leave theinterference region. Each grating structure may act to emit the light asa point wave, and all point-wave fronts emitted from individual gratingstructures are combined into a planar wave front which propagates in aperpendicular direction with low loss. For example, one theoreticalcondition for an ideal match may be d2=2×d1.

Based on the material quality and the physical dimensions of theinterference region and grating structure, an one-circulationattenuation coefficient, α, may be calculated along with a correspondingphase shift for the resonant condition inside the interference region.For example, the interference region may be composed of a material thathas a specific absorption coefficient for the guided light, whichcontributes to the one-circulation attenuation coefficient. As anotherexample, light may be emitted by the grating region during propagation,which also contributes to the one-circulation attenuation coefficient.In general, light is attenuated based on an one-circulation attenuationcoefficient after propagating one-circulation in the interference region(i.e., forward propagation from the boundary 213 to the reflector 211,and then backward propagation from the reflector 211 to the boundary213). The term “one-circulation attenuation coefficient α” as used inthis application refers to a ratio between the remaining optical powerafter the one-circulation attenuation and the initial optical power.

In some implementations, to substantially reduce back reflection loss, areflector region (e.g., the first reflector region 106) may be placed atthe boundary 213, where the reflectivity of the reflector region at theboundary 213 is configured to substantially match the one-circulationattenuation coefficient α. By substantially matching the reflectivity ofthe reflector region at the boundary 213 with α, the light (from theleft-hand side of 213) reflected from the boundary 213 back to theincident light source (at the left-hand side of 213) and the light (fromthe right-hand side of 213) transmitting through the boundary 213 backto the incident light (at the left-hand side of 213) cancel with eachother after multiple passes due to destructive interference, meaningalmost all the power of the original incident light (incident from theleft of 213 into the region between 213 and 211) is transferred into theregion between 213 and 211. In some implementations, the one circulationattenuation coefficient α may be near zero. In this case, thecorresponding reflectivity r at the boundary 213 can be set as zero,which corresponds to the optical coupler 105 in FIG. 1 D, where thefirst reflector region 106 is not included in the optical coupler 105.In some implementations, the reflectivity r at the boundary 213 can beset as high as the reflectivity of the reflector 211, (e.g., close to1), to form a highly confined cavity along the x direction, where thelight can enter or leave the cavity through another direction such as inthe z direction.

In some implementations, there may be non-ideal factors that affect theperformance. For example, a change of effective refractive index mayoccur from the etching of the grating region on the interference region.As another example, the etching process may not create a straight linefrom the grating peak to the grating valley. While the theoreticalmatching condition is d2=2d1, slight deviation from the exact conditionmay be expected during real implementation. Such deviation does notchange the functionality of the optical coupler, but may affect theefficiency. However any reasonable deviation from the ideal case iswithin the scope of this disclosure, where the efficiency is acceptablefor a targeted application. An iterative process of fabricating theoptical coupler, testing the optical coupler, and then re-designing theoptical coupler may improve this issue.

FIG. 3A shows an example of a view of a grating pattern 331 on a planealong the x-y dimensions. The descriptions of FIG. 3A may be applied toany one of the optical couplers described in this application. Thegrating pattern 331 includes an array of one-dimensional gratingstructures 301 a-n and 303 a-n along the x direction, where n is anyinteger greater than one. In some implementations, the gratingstructures 301 a-n and 303 a-n may be composed of different materials.For example, the grating structures 301 a-n may be composed of silicon,and the grating structures 303 a-n may be composed of InP. As anotherexample, the grating structures 303 a-n may include a layer of metalthat forms surface plasmonic effect that couples light from an externalmedium to the interference region. The arrangement of 301 a, 303 a, 301b, 303 b, . . . 301 n, and 303 n forms the grating in a grating region.

FIG. 3B shows an example of a view of a grating pattern 332 on a planealong the x-y dimensions. The descriptions of FIG. 3B may be applied toany one of the optical couplers described in this application. Thegrating pattern 332 includes an array of one-dimensional gratingstructures 305 a-n along the x direction, where n is any integer greaterthan one. In some implementations, the grating structures 305 a-n may begrating peaks of a grating. In some other implementations, the gratingstructures 305 a-n may be grating valleys of a grating. The arrangementof 305 a, 305 b, . . . , and 305 n forms the grating in a gratingregion.

FIG. 3C shows an example of a view of a grating pattern 333 on a planealong the x-y dimensions. The descriptions of FIG. 3C may be applied toany one of the optical couplers described in this application. Thegrating pattern 333 includes an array of two-dimensional rectangulargrating structures 307 a to 307 n along the x direction, and 307 a to307 k along the y direction. In some implementations, the rectangulargrating structure 307 a may be a grating peak of a grating. In someother implementations, the rectangular grating structure 307 a may be agrating valley of a grating. In some implementations, the rectangulargrating structure 307 a may be composed of the same material as thelayer 308, such as silicon. In some implementations, the rectangulargrating structure 307 a may be composed of a different material from thelayer 308. For example, the rectangular grating structure 307 a may becomposed of silicon, and the layer 308 may be composed of InP. In someimplementations, the rectangular grating structure 307 a may be a squareor a non-square, or combinations of both structures. The arrangement ofthe rectangular grating structures 307 a-n and 307 a-k on the x-y planeforms the grating in a grating region. In some implementations, theperiod of the grating along the x direction 321 and period of thegrating along the y direction 322 substantially match the interferencepattern in the layer 308 along the x and y directions, respectively.

FIG. 3D shows an example of a view of a grating pattern 334 on a planealong the x-y dimensions. The descriptions of FIG. 3D may be applied toany one of the optical couplers described in this application. Thegrating pattern 334 includes an array of two-dimensionalarbitrary-shaped grating structures 309 a to 309 n, where n is anyinteger greater than one. In some implementations, the arbitrary-shapedgrating structure 309 a may be a grating peak of a grating. In someother implementations, the arbitrary-shaped grating structure 309 a maybe a grating valley of a grating. In some implementations, thearbitrary-shaped grating structure 309 a may be composed of a differentmaterial from the layer 310. For example, the arbitrary-shaped gratingstructure 309 a may be composed of silicon-dioxide, and the layer 308may be composed of silicon. In some implementations, thearbitrary-shaped grating structure 309 a may be a triangle or an ellipseor combinations of different shapes. The arrangement of thearbitrary-shaped grating structures 309 a-n on the x-y plane forms thegrating in a grating region.

FIG. 3E shows an example of a view of a grating pattern 335 on a planealong the x-y dimensions. The descriptions of FIG. 3E may be applied toany one of the optical couplers described in this application. Thegrating pattern 335 includes an array of two-dimensionalarbitrary-shaped grating structures 313 a to 313n, where n is anyinteger greater than one. In some implementations, the shape of any oneof the arbitrary-shaped grating structures 313 a to 313 n may bedetermined using numerical analysis. For example, a Finite-differencetime-domain (FDTD) analysis program may be used to design the shape ofeach of the arbitrary-shaped structures 313 a to 313n that optimizes thecoupling efficiency. In some implementations, the distance between eachone of the arbitrary-shaped grating structures 313 a to 313 n may bedetermined using numerical analysis. For example, a Finite-differencetime-domain (FDTD) analysis program may be used to determine thedistance between each one of the arbitrary-shaped structures 313 a to313 n that optimizes the coupling efficiency. The arrangement of thearbitrary-shaped grating structures 313 a-n on the x-y plane forms thegrating in a grating region.

In some implementations, the two-dimensional gratings shown in FIG. 3C,FIG. 3D, and FIG. 3E, can have lattice vectors (which define the unitcell size and shape) designed so that the locations of the in-phaseantinodes of the interference region substantially match the locationsof grating valleys and/or peaks.

FIG. 4A shows an example photonic integrated circuit 400 having agrating based optical coupler formed on a light source. The photonicintegrated circuit 400 includes a light source region 430 configured togenerate incident light. In some implementations, the light sourceregion 430 may generate incoherent light. For example, a III-V quantumwell laser diode may include one or more layers of active materials thatgenerate incoherent light when pumped with electrical carriers. In someimplementations, the incoherent light may be coupled to the interferenceregion 410 through spontaneous emission. In some implementations, thelight source region 430 may be confined at other surfaces except thesurface coupled to the interference region 410.

The optical coupler includes a first reflector region 406, a secondreflector region 414, an interference region 410, and a grating region420. The structures of the first reflector region 406, the secondreflector region 414, the interference region 410, the grating region420, and the grating 418 may be implemented by any correspondingstructures described throughout this application, for example, thecorresponding structures in FIGS. 1A-3E. In some implementations, theinterference region 410 and the grating region 420 is composed ofsilicon or III-V materials, the light source region is composed of III-Vmaterials, and the first and second reflector regions 406 and 414include DBR structures or metal coating.

The first reflector region 406 and the second reflector region 414 areconfigured to reflect incident light at a direction that is opposite toa propagation direction of the incident light as indicated by the arrow434. The interference region 410 is formed between the first reflectorregion 406 and the second reflector region 414 and is coupled to thelight source region 430. The interference region 410 may be configuredto (i) guide the light generated by the light source region 430 topropagate along a first direction (the x direction in FIG. 4A), and (ii)confine interference light formed by light reflected between the firstreflector region 406 and the second reflector region 414.

A portion of the light generated in the light source region 430 may becoupled to the interference region 410 through spontaneous emission orany other appropriate coupling mechanism. Light coupled to theinterference region 410 may resonate along the x direction, asdesignated by the arrow 434. Similar to the operations described in FIG.1A, the first reflector region 406 and the second reflector region 414provide reflective surfaces that forms a cavity in the interferenceregion 410, where a standing wave pattern may be formed. Since theinterference region 410 has a fixed cavity length Lcavit_(y), thestanding wave pattern can only be formed by certain wavelengths, and theinterference region 410 may therefore act as a wavelength filter. Insome implementations, incoherent light generated by the light sourceregion 430 may be transformed into coherent light in the interferenceregion 410 by filtering out wavelengths that do not resonate in theinterference region 410.

The grating region 420 includes a grating 418 formed on a regionconfining at least a portion of the interference light. The grating 418is configured to emit a portion of the light at the z direction that issubstantially perpendicular to the x direction. In some implementations,the grating 418 may be designed and fabricated in the grating region 420to substantially match the standing wave pattern in the interferenceregion 410. By matching the standing wave pattern, the grating 418 mayact as an optical antenna and become the most efficient way for light toleave the interference region 410. Each grating structure may act toemit the light as a point wave, and all point-wave fronts emitted fromindividual grating structures are combined into a planar wave frontwhich propagates in the z direction with low loss.

FIG. 4B shows an example photonic integrated circuit 401 having agrating based optical coupler where a light source region 431 is coupledto the interference region 411 by being embedded in the interferenceregion 411. The light source region includes active material layers suchas alternating layers of gallium arsenide (GaAs) and aluminum galliumarsenide (GaAlAs) or alternating layers of InGaAsP and InP. Any othercombinations of active material layers forming a quantum dot, wire, andwell structures that produce incoherent or coherent light are alsowithin the scope of this disclosure.

The interference region 411 is formed between the first reflector region416 and the second reflector region 424. The first reflector region 416and the second reflector region 424 may be formed, for example, bycoating a metal layer or dielectric layers on the end facets or byforming a DBR structure.

Compared to the descriptions of FIG. 4A where the light is generatedoutside the interference region 410, in FIG. 4B, the light is generatedinside the interference region 411. The generated light resonates alongthe x direction between the first reflector region 416 and the secondreflector region 424 in the interference region 411 to generate coherentlight and form a standing wave pattern. The grating region 421 may bedesigned to substantially match the standing wave pattern, where thecoherent light is emitted out of the photonic integrated circuit 401through the grating region 421 along the z direction. In someimplementations, the substrate 440 may be used as a supporting layer. Insome implementations, the substrate 440 may include a DBR structure tofurther reduce the light that propagates in the −z direction.

FIG. 4C shows an example photonic integrated circuit 403 having agrating based optical coupler integrated with a light source, where theinterference light can be controlled by a p-n junction. The photonicintegrated circuit 403 includes a light source region 441, a p-dopedregion 442, an interference region 443, an n-doped region 444, a gratingregion 445, a first reflector region 446, and a second reflector region448. The structures of the light source region 441, the p-doped region442, the interference region 443, the n-doped region 444, the gratingregion 445, the first reflector region 446, and the second reflectorregion 448 may be implemented by any corresponding structures describedthroughout this application, for example, the corresponding structuresin FIGS. 1A-3E.

Similar to the descriptions of FIG. 4A, incoherent light is generated inthe light source region 441, where a portion of the light is coupled tothe interference region 443. The coupled light resonates along the xdirection between the first reflector region 446 and the secondreflector region 448 in the interference region 443 to form a standingwave pattern and generate coherent light. A grating in the gratingregion 445 is designed to substantially match the standing wave pattern,and the coherent light may be emitted out of the photonic integratedcircuit 403 through the grating 443 in the +z or −z direction, dependingon the design of the grating 445.

In some implementations, the n-doped region 444 and a p-doped region 442may be configured to provide an electric field in the interferenceregion 443 with an application of a voltage or a current across then-doped region 444 and the p-doped region 442. The interference region443 may be configured to provide a different interference pattern withthe application of a voltage or a current across the n-doped region 444and the p-doped region 442 due to the generation, recombination,injection or depletion of free carriers. In the case where theinterference pattern is changed due to the change of refractive index,the interference region 443 may stop lasing or may support anotherlasing wavelength. Therefore the application of a voltage or a currentacross the n-doped region 444 and the p-doped region 442 can act as atunable wavelength lasing mechanism or a modulation to the coherentlight.

FIG. 4D shows an example photonic integrated circuit 405 having a lightsource region coupled to an interference region through a waveguideregion where a grating based optical coupler is configured to emit thegenerated light. The photonic integrated circuit 405 includes a lightsource region 450, a waveguide region 451, a first reflector region 452,a second reflector region 453, an interference region 454, and a gratingregion 455. The structures of the light source region 450, the waveguideregion 451, the first reflector region 452, the second reflector region453, the interference region 454, and the grating region 455 may beimplemented by any corresponding structures described throughout thisapplication, for example, the corresponding structures in FIGS. 1A-3E.

Here, incoherent light is generated in the light source region 450,where a portion of the light is coupled to the waveguide region 451. Forexample, the light source region 450 may include an indium phosphide(InP) quantum well (QW) structure bonded to a silicon waveguide, wherethe light generated by the QW is coupled to the silicon waveguide. Thecoupled light resonates between the first reflector region 452 and thesecond reflector region 453 in the interference region 454 to generatecoherent light and form a standing wave pattern. For example, thewaveguide structures of the first reflector region 452 and the secondreflector region 453 may be designed to support a single modepropagation, which may filter out higher order modes in the interferenceregion 454. A grating in the grating region 455 is designed tosubstantially match the standing wave pattern, and the coherent light isemitted out of the photonic integrated circuit 405 through the gratingregion 455 along a direction that is substantially perpendicular to theresonating direction of the light generated by the light source region450. In some implementations, taper regions are used in the interferenceregion to bridge a wider grating region and a narrower waveguide region.The narrow waveguide region may be used to suppress high order mode andthe wide grating region may be used to match the external couplingdevice with different beam shapes areas and numerical aperturerequirement . The descriptions of the taper regions may be applied toany one of the optical couplers described in this application.

FIG. 4E shows an example photonic integrated circuit 407 having a lightsource region coupled to a grating based optical coupler through thegrating region 466. The photonic integrated circuit 407 includes a lightsource region 462, a first reflector region 478, a second reflectorregion 476, an interference region 472, a boundary 474, and a gratingregion 466. The structures of the light source region 462, the firstreflector region 478, the second reflector region 476, the interferenceregion 472, and the grating region 466 may be implemented by anycorresponding structures described throughout this application, forexample, the corresponding structures in FIGS. 1A-3E.

Here, incoherent light is generated in the light source region 462 andcoupled to the interference region 472 through spontaneous emission oranother suitable coupling mechanism. The coupled incoherent light isreflected by the reflector regions 476 and 478, and resonates along thex direction. The coherent light, when reaching lasing threshold, canemit either in the +z or −z direction depending on the design of thegrating region 466. In some implementations, the grating region 466 canbe designed to direct light mostly to the −z direction so that theemitted light does not couple back to the light source region 462.

In some implementations, the reflector 478 can be a partial reflectorwith less reflectivity than the reflector 476. The incoherent lightgenerated in the light source region 462 can couple to the interferenceregion 472 through spontaneous emission, and resonates along thedirection 470. When reaching the lasing threshold, the coherent lightcan propagate in the −x direction through the boundary 474 into thepartial reflector 478 and then into a waveguide, and at the same timepropagate in the +z or −z direction into an external medium for furtherprocessing.

FIG. 4F shows an example of the photonic circuit 408 having similarlayer stacks as FIG. 4B. The interference region 491 is composed ofactive materials such as III-V materials to generate incoherent light.The incoherent light resonates between the first reflector 486 and thesecond reflector 484 along the x direction and becomes coherent. Thecoherent light is emitted through the grating 492 upward since the DBRstructure 494 is used to reduce the light that propagates in thedownward direction.

FIG. 5A shows an example photonic integrated circuit 500 having anoptical coupler integrated with a light detector. The photonicintegrated circuit 500 includes an interference region 510, a firstreflector region 506, a second reflector region 514, and a gratingregion 520. The interference region 510 includes a light absorptionregion that is configured to absorb at least a portion of the light inthe interference region 510. The structures of the interference region510, the first reflector region 506, the second reflector region 514,and the grating region 520 may be implemented by any correspondingstructures described throughout this application, for example, thecorresponding structures in FIGS. 1A-3E. The photonic integrated circuit500 may be formed on a substrate 530.

The grating region 520 includes a grating 518. In some implementations,the grating 518 may be configured to receive, from an external medium,light propagating in the direction indicated by the arrow 536. Light maybe coupled to the interference region 510 formed between the firstreflector region 506 and the second reflector region 514. Theinterference region 510 is configured to confine interference lightformed by light reflected by the first reflector region 506 and thesecond reflector region 514 along the direction indicated by the arrow534.

The interference region 510 may be composed of a light absorptionmaterial that absorbs at least a portion of the interference light. Forexample, the interference region 510 may be composed of germanium thatis configured to absorb light in telecommunications wavelengths. Theabsorbed light is converted to free carriers, which can be collectedwith an applied electrical field and used to determine an optical powerlevel of the received light. In addition, light is reflected in a cavityformed by the first reflector region 506 and the second reflector region514, providing an effective optical absorption. In some implementations,the interference region 510 includes silicon and/or germanium for lightabsorption and the grating region includes metal to create guidedsurface plasma modes.

FIG. 5B shows an example photonic integrated circuit 501 having agrating based optical coupler integrated with a light detector, wherethe light detection efficiency can be modulated by a p-n junction. Thephotonic integrated circuit 501 includes an interference region 541, ap-doped region 546, an n-doped region 545, a grating 542, a firstreflector region 543, and a second reflector region 544.

Similar to the descriptions of FIG. 5A, light is received from anexternal medium, where a portion of the light is coupled to theinterference region 541 through the grating 542. The coupled lightresonates along the x direction between the first reflector region 543and the second reflector region 544 in the interference region 541. Thelight absorbing material in the interference region 541 absorbs theinterference light and coverts the light into free carriers.

In some implementations, the n-doped region 545 and a p-doped region 546may be configured to provide an electric field in the interferenceregion 541 with an application of a voltage or a current across then-doped region 545 and the p-doped region 546. The absorption efficiencyof the light absorbing material in the interference region 541 may varybased on the application of the voltage or a current across the n-dopedregion 545 and the p-doped region 546. Therefore the application of avoltage or a current across the n-doped region 545 and the p-dopedregion 546 acts as a configurable or tunable photo-detector.

FIG. 5C shows an example photonic integrated circuit 505 having a lightabsorption region coupled to an interference region through a waveguideregion, where a grating based optical coupler is configured to receivelight from an external medium. The photonic integrated circuit 505includes a light absorption region 550, a waveguide region 551, a firstreflector region 552, a second reflector region 553, an interferenceregion 554, and a grating region 555. The structures of the lightabsorption region 550, the waveguide region 551, the first reflectorregion 552, the second reflector region 553, the interference region554, and the grating region 555 may be implemented by any correspondingstructures described throughout this application, for example, thecorresponding structures in FIGS. 1A-3E.

Here, light is coupled to the grating region 555 from an externalmedium. The coupled light resonates between the first reflector region552 and the second reflector region 553 in the interference region 554to form a standing wave pattern. A light absorption region 550 iscoupled to the waveguide region 551, and light is coupled from thewaveguide region 551 to the light absorption region 550 for detection.For example, the light absorption region 550 may be a germaniumphoto-detector bonded on a silicon waveguide, where light propagating inthe silicon waveguide is confined in the interference region 554 and thewaveguide region 551, and evanescently coupled to the germaniumphoto-detector.

FIG. 6 shows an example of an optical coupler 600 integrated with p-njunctions. The optical coupler 600 includes a first reflector region606, an interference region 620, and a second reflector region 614. Theinterference region 620 includes a grating region 630. The firstreflector region 606, the interference region 620, the second reflectorregion 614, and the grating region 630 may be implemented using any oneof the corresponding regions as described throughout the application.

The optical coupler 600 also includes p-n junction pairs includingp-doped regions 621, 623, and 625, and n-doped regions 631, 633, and635. In general, by controlling one or more p-n junction pairs,parameters such as output power and output wavelength may be activelycontrolled by the application of voltages or carriers injection. In someimplementations, the p-n junction pairs 621/631, 623/633, and/or 625/635may extend into the first reflector region 606, the interference region620, and/or the second reflector region 614, respectively, for bettercontrollability. In some implementations, the p-doped and n-dopedregions may alternate to form interdigitated patterns or other patterns.The descriptions of the doped regions may be applied to any one of theoptical couplers described in this application.

In some implementations, an n-doped region 631 and a p-doped region 621may be configured to provide an electric field in the first reflectorregion 606 with an application of a voltage or a current across then-doped region 631 and the p-doped region 621, where the first reflectorregion 606 may be configured to provide a different reflectivity withthe application of the voltage or a current across the n-doped region631 and the p-doped region 621.

In some implementations, an n-doped region 635 and a p-doped region 625may be configured to provide an electric field in the second reflectorregion 614 with an application of a voltage or a current across then-doped region 635 and the p-doped region 625, wherein the secondreflector region 614 may be configured to provide a differentreflectivity with the application of a voltage or a current across then-doped region 635 and the p-doped region 625.

In some implementations, an n-doped region 633 and a p-doped region 623may be configured to provide an electric field in the interferenceregion 620 with an application of a voltage or a current across then-doped region 633 and the p-doped region 623, wherein the interferenceregion 620 may be configured to provide a different interference patternfor the interference light with the application of a voltage or acurrent across the n-doped region 633 and the p-doped region 623.

For example, by applying a reverse bias voltage, the electric field mayextract the free carriers in a region, and therefore may modify therefractive index of the region. As another example, by applying aforward bias voltage, free carriers may be injected into a region, andtherefore may modify the refractive index of the region.

FIG. 7A shows an example photonic integrated circuit 700 having multipleoutputs. The photonic integrated circuit 700 includes a first waveguideregion 702 configured to guide line in the direction as designated bythe arrow 722. The photonic integrated circuit 700 includes a firstgrating region 720 formed on a side of the interference region 710. Thephotonic integrated circuit 700 includes a second grating region 721formed on a different side of the interference region 710, for example,the opposite side as shown in FIG. 7A. The photonic integrated circuit700 includes a reflector region 714, and may optionally include anotherreflector region 706. The photonic integrated circuit 700 includes asecond waveguide region 728 that may be coupled to other passive and/oractive optical components.

In some implementations, light from the first waveguide region 702enters the interference region 710, and can be directed to a firstexternal medium 730, a second external medium 732, or a second waveguideregion 728. For example, similar to the descriptions of FIG. 6, ann-doped region and a p-doped region may be configured to provide anelectric field in the interference region 710 with an application of avoltage or a current across the n-doped region and the p-doped region,where the portion of the light emitted in the +z direction and theportion of the light emitted in the −z direction may be controlled bythe application of a voltage or a current across the n-doped region andthe p-doped region. As another example, similar to the descriptions ofFIG. 6, an n-doped region and a p-doped region may be configured toprovide an electric field in the second reflector region 714 with anapplication of a voltage or a current across the n-doped region and thep-doped region. The reflectivity of the second reflector region 714 maybe tuned, and light may be transmitted to the second waveguide region728.

In some implementations, the light enters the interference region 710,and can be split into different portions of light that exit for thefirst external medium 730, the second external medium 732, and/or thesecond waveguide region 728. For example, the grating in the firstgrating region 720 may be designed such that the grating periodicitysubstantially matches the standing wave of a TE-polarized light.Similarly, the grating in the second grating region 721 may be designedsuch that the grating periodicity substantially matches the standingwave of a TM-polarized light. By controlling the proportion of TE and TMpolarized light in the photonic integrated circuit 700, the portions oflight exiting the photonic integrated circuit 700 for the first externalmedium 730 and the second external medium 732 may be controlled. Theabove example can serve as an effective polarization beam splitter.

In some implementations, a first layer 724 may be formed between thefirst grating region 720 and the first external medium 730. The firstlayer 724 may be formed to protect the photonic integrated circuit 700,or to provide a specific distance between the first grating region 720and the first external medium 730. In some implementations, a secondlayer 716 may be formed between the second grating region 721 and thesecond external medium 732. The second layer 716 may be formed toprotect the photonic integrated circuit 700, or to provide a specificdistance between the second grating region 721 and the second externalmedium 732. For example, the first layer 724 may be a cladding, and thesecond layer 716 may be a substrate of the photonic integrated circuit700. As another example, the first layer 724 may have a lower refractiveindex compared to the grating region 720.

FIG. 7B shows an example photonic integrated circuit 701 having multipleinputs and outputs. The photonic integrated circuit 701 may include afirst waveguide region 751, a second waveguide region 752, a thirdwaveguide region 753, a fourth waveguide region 754, p-n junctions755-760 and 795-798, a first reflector region 761, a second reflectorregion 762, a third reflector region 791, a fourth reflector region 792,an interference region 770, and a two-dimensional grating 780. Thestructures of the first waveguide region 751, the second waveguideregion 752, the third waveguide region 753, the fourth waveguide region754, the p-n junctions 755-760 and 795-798, the first reflector region761, the second reflector 762, the third reflector region 791, thefourth reflector region 792, the interference region 770, and thetwo-dimensional grating 780 may be implemented by any correspondingstructures described throughout this application, for example, thecorresponding structures in FIGS. 1A-6.

In some implementations, light from the first and the third waveguideregion 751 and 753 enter the interference region 770, and may bedirected to the second waveguide region 752, the fourth waveguide region754, or out of the grating in the z direction as designated by theoutward arrow 764. In some implementations, to minimize the backreflection of the input light from the first and the third waveguideregions, the reflectivity of the first and the third reflector regions761 and 791 can be tuned during the initial design or through dynamicapplication of electrical field to match the one-circulation attenuationcoefficients of the wave propagating along the x and y directionrespectively.

In some implementations, light from the first waveguide region 751enters the interference region 770, and may be split to differentportions to the second waveguide region 752, the third waveguide region753, the fourth waveguide region 754, and/or out of the grating in the zdirection as designated by the outward arrow 764. For example, thegrating region may include a two-dimensional grating 780 configured toseparate light into two light portions propagating along two directionsx and y according to wavelengths. As another example, the grating regionmay include a two-dimensional grating 780 configured to separate lightinto two light portions propagating along two directions x and yaccording to polarization. As another example, the two-dimensionalgrating 780 may be configured to reversely combine two light portionspropagating along two directions x and y into one light portion.

In some implementations, the two-dimensional grating 780 may beconfigured to emit light upward in the z direction by combining twolights coming from two directions x and y, by substantially matching theperiods of the grating 780 along the x and y direction to periods of theinterference pattern along the x and y directions respectively. In someimplementations, the two-dimensional grating 780 may be configured toemit light upward in the z direction by combining two lights coming fromtwo directions x and y, by modifying the interference pattern along thex and y directions, through the application of electrical fields betweenp-doped and n-doped regions, to match the grating 780 pattern along thex and y directions respectively. As another example, the two-dimensionalgrating 780 may be configured to reversely split one light propagatingat one direction (e.g. −z) into two light portions propagating along twodirections x and y.

Similar to the descriptions of FIG. 7A, in some implementations, ann-doped region and a p-doped region may be configured to provide anelectric field in the interference region with an application of avoltage or a current across the n-doped region and the p-doped region,where a respective proportion of each of the two light portionspropagating along two directions is controlled by the application of avoltage or a current across the n-doped region and the p-doped region.For example, a voltage may be applied across the n-doped region 759 andthe p-doped region 756 to control the routing or the splitting of theinput light. In some implementations, one or more doped regions can beeliminated if there is no need to modulate the refractive index ofcorresponding regions. For example, the doped regions 756 and 759 can beeliminated if there is no need to dynamically modulate the interferenceregion 770.

FIG. 8 shows an example of a photonic integrated circuit 800 suitablefor packaging. The photonic integrated circuit 800 includes an edgeemitting light source 801 that is butt-coupled to a waveguide 803. Thelight may be modulated by optionally including an integrated modulator805 as shown in FIG. 8, or through direct modulation of the edgeemitting light source 801. The modulated light is coupled to agrating-based optical coupler 807 as described in previous figures.Since light emitted by the optical coupler 807 may be designed to emitalong a direction that is substantially perpendicular than the directionof light emitted by the edge emitting light source 801, the photonicintegrated circuit 800 may be readily tested and packaged with astandardized optical equipment. For example, the die where the photonicintegrated circuit 800 is fabricated on may be attached to the insidesurface of a transistor outline (TO) metal can, where the grating may bealigned to the aperture of the TO metal can, allowing verticaltransmission of light out of the package.

FIG. 9A shows an example of an optical apparatus 901, including a cornermirror 902 that provides a high-reflectivity. The descriptions of FIG.9A may be applied to any one of the reflector regions disclosed in thisapplication. In general, light propagates through the grating region920, and is incident on the facets 931 a and 931 b of the corner mirror902. Because the light is incident on the facets 931 a and 931 b beyonda total reflection angle, substantial light is reflected due to totalreflection, achieving a high-reflectivity.

FIG. 9B shows an example of an optical apparatus 903, including acircular or elliptical facet 904 that provides a partial-reflectivity ora high-reflectivity. The descriptions of FIG. 9B may be applied to anyone of the reflector regions disclosed in this application. In general,light propagates through the curved grating region 921, and is incidenton the facet 904. In some implementations, the facet 904 may be coatedwith a metal layer to provide a high-reflectivity. The curved facet 904refocuses the light back toward the direction of the waveguide region bythe high-reflectivity curved facet 904.

FIG. 9C shows an example of an optical apparatus 905, including adistributed Bragg reflector (DBR) mirror 906 providing ahigh-reflectivity. The descriptions of FIG. 9C may be applied to any oneof the reflector regions disclosed in this application. In general,light propagates through the grating region 922, and is incident on DBRmirror 906. In some implementations, the DBR mirror 906 may be designedto provide a high reflectivity for a range of wavelengths.

FIG. 9D shows an example of an optical apparatus 907, including a facet908 providing a partial-reflectivity or a high-reflectivity. Thedescriptions of FIG. 9D may be applied to any one of the reflectorregions disclosed in this application. In general, light propagatesthrough the grating region 923, and is incident on the facet 908.Without any additional coating, the reflectivity may be determined usingthe Fresnel equations. In some implementations, the facet 908 may becoated with one or more layers of materials to increase thereflectivity. For example, the facet 908 may be coated with a metallayer to increase the reflectivity. As another example, the facet 908may be coated with multiple dielectric layers to increase thereflectivity for a range of wavelengths. As another example, the facet908 may be coated with a quarter-wavelength dielectric layer followed bya metal layer to increase the reflectivity for a range of wavelengths.

FIG. 9E shows an example of an optical apparatus 909, including asurface corrugated mirror 936. The descriptions of FIG. 9E may beapplied to any one of the reflector regions disclosed in thisapplication. In general, light propagates through the grating region932, and is incident into the mirror 936 and gets reflected, forming aninterference pattern.

Any other types of reflectors that can be integrated in a photonicintegrated circuit may also be used as alternatives to the reflectorsdescribed in FIGS. 9A-9E. For example, a reflector region mayalternatively include an anomalously dispersive mirror or a waveguideloop mirror.

FIG. 10 shows an example of a flow diagram for designing an opticalcoupler. The process 1000 may be performed by data processing apparatus,such as one or more computers.

The system obtains cavity dimensions (1002). In some implementations,based on the target light polarization/mode/wavelength/spot size, andthe external medium (e.g., fiber on top of the grating or waveguideconnected to the second side 114, etc.), the dimensions and materials ofthe cavity and the substrate can be determined. For example, for asingle mode optical signal with center wavelength around 1310 nm, a Silayer cavity having a thickness of 250 nm on an oxide layer can be used.If the spot size of the external fiber is around 10 μm, the dimension ofthe cavity needs to be around or larger than 10 μm to allow the externalfiber to be coupled onto the grating structure.

The system determines grating dimensions (1004). In general, the designof the grating dimensions depends on the standing wave pattern insidethe cavity. The period of the grating pattern needs to be substantiallymatched to the standing wave pattern, and the other parameters, such asthe duty cycle, and grating height, shape of the grating structure, areother design parameters that may be optimized based on parameters suchas the incident light spot size, mode of the light, wavelength of thelight, material of the cavity, intended emitting angle, etc. Thestanding wave pattern can be estimated assuming a specific mirrorreflectivity.

The system determines grating characteristics (1006). For example, anumerical analysis tool such as a FDTD simulation system can be used tooptimize the design parameters until a high directionality and theintended emission angle, for example a substantially vertical emission,are both reached.

The system determines mirror characteristics (1008). In someimplementations, a numerical analysis tool such as a FDTD simulationsystem may optimize design parameters to generate a design for a backmirror having a reflection that is close to 100%. For example, thedesign may be a tapered waveguide distributed Bragg reflector (DBR), awaveguide loop mirror, or a Si—SiO2-metal coating layer.

In some implementations, the FDTD simulation system may be used todetermine the one-circulation attenuation coefficient α, as described inFIG. 2. Based on the calculated one-circulation attenuation coefficientα, the system may design a front mirror having a reflectivity thatsubstantially matches the one-circulation attenuation coefficient α. Insome implementations, multiple doped regions can be placed near theregions to modify the properties of the regions through the applicationof electrical field.

In some implementations of the design flow, after obtaining the cavitydimensions (1002), a proper mirror design (e.g., tapered DBR or cornermirror or oxide-metal coating, etc.) can be determined to form theinterference wave pattern. Then, grating structure on top of theinterference region can be designed based on the initial interferencewave pattern. By adding the grating structure, characteristics of theinterference region may change, resulting in a modification of theinterference wave pattern in the interference region. Therefore, someiteration processes might be needed for optimization. Based on thematerial quality and the physical dimensions of the interference regionand the grating, the one circulation attenuation coefficient (α) can bedetermined along with the corresponding phase shift for the resonantcondition. Then another mirror or reflector can be added on another sideof the interference region.

FIG. 11 shows an example of a flow diagram for fabricating an opticalcoupler. The process 1100 may be performed by a system including dataprocessing apparatus, such as one or more computers that control one ormore apparatuses that perform the fabrication steps.

The system fabricates gratings (1102). The fabrication of gratings maybe done by a combination of CMOS compatible fabrication techniques. Forexample, lithography techniques such as projection lithography,electronic-beam lithography, contact lithography, or any other suitablelithography techniques may be used to pattern the gratings. As anotherexample, etching techniques such as dry etching, wet etching, or anyother suitable etching techniques may be used to etch the patternedgratings. As another example, thin film deposition techniques such aschemical vapor deposition, plasma-enhanced chemical vapor deposition,sputtering, or any other suitable thin film deposition techniques may beused to deposit one or more layers of materials on the gratings.

The system can also optionally measure grating characteristics (1104) asa mid-process check point. In some implementations, the measurement ofgrating characteristics may be performed on a die level. In someimplementations, the measurement of grating characteristics may beperformed on a wafer level. In some implementations, input light iscoupled into a photonic integrated circuit through a fabricated gratingon the input end, and output light is coupled out of the photonicintegrated circuit through another fabricated grating on the output end.In some implementations, input light is coupled into a photonicintegrated circuit through a fabricated grating, where the light isreflected by a high-reflectivity reflector. The reflected light may becoupled out of the photonic integrated circuit through the same grating,and separated out by an optical circulator. The grating characteristicssuch as the one-circulation attenuation coefficient may be measured andanalyzed by comparing the output light with the input light. In someimplementations, the measurement of grating characteristics may beperformed by shining a light at a tilt angle onto the fabricated gratingand measure the reflected light at another angle.

The system fabricates mirrors (1106). In some implementations, after thesystem determines the one-circulation attenuation coefficient, thesystem can optimize the parameters for a reflector that matches theone-circulation attenuation coefficient. The fabrication of the one ormore reflectors may be done by a combination of CMOS compatiblefabrication techniques. In some implementations, the doped regions canbe lithography defined, implanted and activated at locations near theregions needs to be modified through the application of electricalfield.

A number of implementations have been described. Nevertheless, it willbe understood that various modifications may be made without departingfrom the spirit and scope of the disclosure. For example, various formsof the flows shown above may be used, with steps re-ordered, added, orremoved. For example, in FIG. 10, step 1008 can also be done before step1004. As another example, in FIG. 11, step 1104 can also be done afterstep 1106.

Various implementations may have been discussed using two-dimensionalcross-sections for easy description and illustration purpose.Nevertheless, the three-dimensional variations and derivations shouldalso be included within the scope of the disclosure as long as there arecorresponding two-dimensional cross-sections in the three-dimensionalstructures.

Embodiments and all of the functional operations described in thisspecification may be implemented in digital electronic circuitry, or incomputer software, firmware, or hardware, including the structuresdisclosed in this specification and their structural equivalents, or incombinations of one or more of them. Embodiments may be implemented asone or more computer program products, i.e., one or more modules ofcomputer program instructions encoded on a computer-readable medium forexecution by, or to control the operation of, data processing apparatus.The computer readable-medium may be a machine-readable storage device, amachine-readable storage substrate, a memory device, a composition ofmatter affecting a machine-readable propagated signal, or a combinationof one or more of them. The computer-readable medium may be anon-transitory computer-readable medium. The term “data processingapparatus” encompasses all apparatus, devices, and machines forprocessing data, including by way of example a programmable processor, acomputer, or multiple processors or computers. The apparatus mayinclude, in addition to hardware, code that creates an executionenvironment for the computer program in question, e.g., code thatconstitutes processor firmware, a protocol stack, a database managementsystem, an operating system, or a combination of one or more of them. Apropagated signal is an artificially generated signal, e.g., amachine-generated electrical, optical, or electromagnetic signal that isgenerated to encode information for transmission to suitable receiverapparatus.

A computer program (also known as a program, software, softwareapplication, script, or code) may be written in any form of programminglanguage, including compiled or interpreted languages, and it may bedeployed in any form, including as a standalone program or as a module,component, subroutine, or other unit suitable for use in a computingenvironment. A computer program does not necessarily correspond to afile in a file system. A program may be stored in a portion of a filethat holds other programs or data (e.g., one or more scripts stored in amarkup language document), in a single file dedicated to the program inquestion, or in multiple coordinated files (e.g., files that store oneor more modules, sub programs, or portions of code). A computer programmay be deployed to be executed on one computer or on multiple computersthat are located at one site or distributed across multiple sites andinterconnected by a communication network.

The processes and logic flows described in this specification may beperformed by one or more programmable processors executing one or morecomputer programs to perform functions by operating on input data andgenerating output. The processes and logic flows may also be performedby, and apparatus may also be implemented as, special purpose logiccircuitry, e.g., an FPGA (field programmable gate array) or an ASIC(application specific integrated circuit).

Processors suitable for the execution of a computer program include, byway of example, both general and special purpose microprocessors, andany one or more processors of any kind of digital computer. Generally, aprocessor will receive instructions and data from a read only memory ora random access memory or both. The essential elements of a computer area processor for performing instructions and one or more memory devicesfor storing instructions and data. Generally, a computer will alsoinclude, or be operatively coupled to receive data from or transfer datato, or both, one or more mass storage devices for storing data, e.g.,magnetic, magneto optical disks, or optical disks. However, a computerneed not have such devices. Moreover, a computer may be embedded inanother device, e.g., a tablet computer, a mobile telephone, a personaldigital assistant (PDA), a mobile audio player, a Global PositioningSystem (GPS) receiver, to name just a few. Computer readable mediasuitable for storing computer program instructions and data include allforms of non-volatile memory, media and memory devices, including by wayof example semiconductor memory devices, e.g., EPROM, EEPROM, and flashmemory devices; magnetic disks, e.g., internal hard disks or removabledisks; magneto optical disks; and CD-ROM and DVD-ROM disks. Theprocessor and the memory may be supplemented by, or incorporated in,special purpose logic circuitry.

To provide for interaction with a user, embodiments may be implementedon a computer having a display device, e.g., a CRT (cathode ray tube) orLCD (liquid crystal display) monitor, for displaying information to theuser and a keyboard and a pointing device, e.g., a mouse or a trackball,by which the user may provide input to the computer. Other kinds ofdevices may be used to provide for interaction with a user as well; forexample, feedback provided to the user may be any form of sensoryfeedback, e.g., visual feedback, auditory feedback, or tactile feedback;and input from the user may be received in any form, including acoustic,speech, or tactile input.

Embodiments may be implemented in a computing system that includes aback end component, e.g., as a data server, or that includes amiddleware component, e.g., an application server, or that includes afront end component, e.g., a client computer having a graphical userinterface or a Web browser through which a user may interact with animplementation of the techniques disclosed, or any combination of one ormore such back end, middleware, or front end components. The componentsof the system may be interconnected by any form or medium of digitaldata communication, e.g., a communication network. Examples ofcommunication networks include a local area network (“LAN”) and a widearea network (“WAN”), e.g., the Internet.

The computing system may include clients and servers. A client andserver are generally remote from each other and typically interactthrough a communication network. The relationship of client and serverarises by virtue of computer programs running on the respectivecomputers and having a client-server relationship to each other.

While this specification contains many specifics, these should not beconstrued as limitations, but rather as descriptions of featuresspecific to particular embodiments. Certain features that are describedin this specification in the context of separate embodiments may also beimplemented in combination in a single embodiment. Conversely, variousfeatures that are described in the context of a single embodiment mayalso be implemented in multiple embodiments separately or in anysuitable subcombination. Moreover, although features may be describedabove as acting in certain combinations and even initially claimed assuch, one or more features from a claimed combination may in some casesbe excised from the combination, and the claimed combination may bedirected to a subcombination or variation of a subcombination.

Similarly, while operations are depicted in the drawings in a particularorder, this should not be understood as requiring that such operationsbe performed in the particular order shown or in sequential order, orthat all illustrated operations be performed, to achieve desirableresults. In certain circumstances, multitasking and parallel processingmay be advantageous. Moreover, the separation of various systemcomponents in the embodiments described above should not be understoodas requiring such separation in all embodiments, and it should beunderstood that the described program components and systems maygenerally be integrated together in a single software product orpackaged into multiple software products.

Thus, particular embodiments have been described. Other embodiments arewithin the scope of the following claims. For example, the actionsrecited in the claims may be performed in a different order and stillachieve desirable results.

What is claimed is:
 1. (canceled)
 2. An optical apparatus, comprising: alight source region configured to generate light; a first reflectorregion and a second reflector region; an interference region formedbetween the first reflector region and the second reflector region, theinterference region configured to receive a portion of the lightgenerated by the light source region and to confine light reflectedbetween the first reflector region and the second reflector region alonga first direction; and a grating region comprising a first gratingstructure having a first grating pattern, the first grating structureformed on a region confining at least a portion of the interferencelight, the first grating structure configured to emit at least a portionof the light along a second direction that is different from the firstdirection, wherein a first grating periodicity of the first gratingpattern substantially matches an interference periodicity of the lightinside the interference region.
 3. The optical apparatus of claim 2,wherein the grating region further includes a second grating structurehaving a second grating pattern with a second grating periodicity, andthe first grating structure and the second grating structure arearranged along the first direction with 180 degree phases offset.
 4. Theoptical apparatus of claim 3, wherein the second grating periodicitysubstantially matches the first grating periodicity.
 5. The opticalapparatus of claim 3, wherein a first grating duty cycle of the firstgrating structure is different from a second grating duty cycle of thesecond grating structure.
 6. The optical apparatus of claim 2, whereinthe first reflector region is a surface corrugated grating structureforming a distributed feedback or a distributed Bragg reflector.
 7. Theoptical apparatus of claim 6, wherein the first grating structure andthe surface corrugated grating structure are formed on a planar surface.8. The optical apparatus of claim 6, wherein a duty cycle of the firstgrating structure is different from a duty cycle of the surfacecorrugated grating structure.
 9. The optical apparatus of claim 2,wherein the light source is at least partially embedded in theinterference region, and contains alternating III-V material layersforming a quantum well or quantum dot structure.
 10. The opticalapparatus of claim 2, wherein a portion of the grating region is formedon the interference region, the first reflector region, or the secondreflector region.
 11. The optical apparatus of claim 2, wherein thesecond direction is substantially perpendicular to the first direction.12. The optical apparatus of claim 2, wherein the first grating patternincludes a grating periodicity of d2, wherein the standing wave patternincludes an intensity periodicity of d1, wherein d2 substantially equalsto 2×d1.
 13. The optical apparatus of claim 2, further comprising: ann-doped region and a p-doped region configured to provide an electricfield in the interference region with an application of a voltage or acurrent across the n-doped region and the p-doped region, wherein theinterference region is configured to provide a different standing wavepattern with the application of a voltage or a current across then-doped region and the p-doped region.
 14. The optical apparatus ofclaim 2, further comprising: an n-doped region and a p-doped regionconfigured to provide an electric field in the first reflector region orthe second reflector region with an application of a voltage or acurrent across the n-doped region and the p-doped region, wherein thefirst reflector region or the second reflector region is configured toprovide a different reflectivity with the application of a voltage or acurrent across the n-doped region and the p-doped region.
 15. Theoptical apparatus of claim 2, further comprising an optical packagehaving an aperture arranged to allow light to emit along the seconddirection.
 16. The optical apparatus of claim 2, wherein the firstreflector region or the second reflector region includes one of a cornermirror, a DBR mirror, a DFB mirror, an anomalously dispersive mirror, awaveguide loop mirror, or a metal layer.